1. Field of the Invention
The present invention relates to an optical recording medium including a plurality of recording and reading layers, and a series of optical recording media made by preparing optical recording media of a plurality of types.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, DVD-RAMs, Blu-ray Discs (BD) are widely used to view digital video contents and to record digital data. In the BD standard, which is one of the next generation DVD standards, the wavelength of the laser beam used for recording and reading is reduced to 405 nm, and the numerical aperture of an objective lens is set to 0.85. In optical recording media conforming to the BD standard, tracks are formed at 0.32 μm pitch. This enables 25 GB or more data to be recorded on and read from one recording and reading layer of such an optical recording medium.
It is expected that the size of video and data files will increase more and more in the future. Therefore, it is contemplated to increase the capacities of optical recording media by using a multiple stack of recording and reading layers. In the technologies for BD standard optical recording media reported by I. Ichimura et al., Appl. Opt., 45, 1974-1803 (2006) and K. Mishima et al., Proc. of SPIE, 6282, 62820I (2006), 6 to 8 recording and reading layers are provided to achieve ultra large capacities as much as 200 GB.
In a multilayer optical recording medium, crosstalk of a signal of a different recording and reading layer, or of noise is generally generated during recording and reading of a target recording and reading layer. This results in degradation of the quality of a servo signal or a recording signal.
The aforementioned crosstalk includes two types: interlayer crosstalk and confocal crosstalk. The interlayer crosstalk is a phenomenon generated by interference of light reflected from a recording and reading layer adjacent to a recording and reading layer being read into reading light. Accordingly, this crosstalk is always a matter of concern in an optical recording medium including two or more recording and reading layers. Increasing an interlayer distance reduces the interlayer crosstalk. However, increasing an interlayer distance in turn makes it impossible to increase the number of recording and reading layers.
The confocal crosstalk is specific to an optical recording medium including three or more layers. The confocal crosstalk is a phenomenon generated by the fact that main reading light reflected from a target recording and reading layer, and stray light reflected from a different recording and reading layer a plurality of times have the same optical path length. The intensity of stray light determined by the product of reflectances of reflections decreases with a larger number of reflections. Accordingly, for practical purposes, considering the case of three reflections is sufficient. A technique of making all interlayer distances (thicknesses of intermediate layers) different is known to prevent the confocal crosstalk.
However, making the thicknesses of intermediate layers different from each other in order to increase the number of recording and reading layers requires intermediate layers of various thicknesses. This only results in a greater intermediate distance. Thus, a recording and reading layer farthest from a light incident surface goes farther from the light incident surface, causing an adverse effect on a coma aberration due to a tilt and the like.
Furthermore, in the optical recording medium of this type, projections and depressions, such as grooves and lands, for tracking control should be formed in each recording and reading layer. Thus, projections and depressions should be formed in each intermediate layer with a stamper, so that an error is likely to be generated in the thicknesses of the intermediate layers. Such an error in film deposition should be considered if the thicknesses of the intermediate layers are to be made different from each other as described above. This disadvantageously increases the thickness of the optical recording medium further.
The present invention has been made in view of the above problems, and it is an object of the invention to provide an optical recording medium including three or more layers, the optical recording medium being capable of reasonably realizing reduction of an interlayer distance and reduction of crosstalk at the same time.